U.S. Pat. No. 4,634,918 describes an example of a liquid cooling type cathode-ray tube which comprises a cathode-ray tube assembly, a metal die cast coupler with a projection lens held thereon, the coupler being attached to the front surface of the cathode-ray tube assembly, and a coolant sealed in a liquid tight space formed by the front surface of the cathode-ray tube assembly, the coupler and the projection lens. In such cathode-ray tube, an earth potential is applied to an outer conductive film (e.g., carbon film) coated on the outer side surface of a funnel. The earth potential is generally applied to the outer conductive film by conventional methods using an earth spring or an earth wire.
Further, there is proposed a method of using an earth wire in the liquid cooling type cathode-ray tube in which the coupler is provided with protruded portions and an earth wire that comes in contact with the outer conductive film is extended between the protrusion portions. However, if the protruded portions are round, then the earth wire is easily detached from the protruded portions. If the protruded portions have stepped portions in order to prevent the earth wire from being detached therefrom, then a metal mold for die-casting such protruded portions become complicated in structure, and the protruded portions become difficult to be molded, and expensive.
Furthermore, there is proposed a method in which a coupler or the like has a threaded portion and an earth wire which comes in contact with an outer conductive film is fixed by screws. This proposed method has the increased number of processes such as threading or screwing, which results in a poor working property. In addition, a liquid cooling type cathode-ray tube become expensive due to a processing cost of threading and costs of screws.